1. Technical Field
The present disclosure is directed to electrosurgery and, in particular, to an electrosurgical generator capable of controlling its output crest factor, as well as the output power, across a range of tissue impedances during electrosurgery.
2. Description of the Related Art
Tissue heating is proportional to the square of the amount of current being generated through the tissue and tissue vaporization is, in turn, generally proportional to current. Vaporization of tissue is proportional to the amount of energy in an arc. This energy in combination with the Cathode Fall Voltage, derives the power of vaporization. Thermal spread is dependent on the amount of heat generated within the tissue which is dependent on tissue resistivity and the arc energy squared. As can be appreciated, by not controlling the thermal spread the depth of ablation is difficult to predict and control.
Therefore, during electrosurgery, an increase or decrease in the amount of current provides a different tissue effect. This phenomenon is due to a variable referred to as the crest factor (CF). The crest factor can be calculated using the formula: CF=VPEAK/VRMS, where VPEAK is the positive peak of the waveform and VRMS is the RMS value of the waveform. The crest factor can also be calculated using the formula: CF=[(1−D)/D]1/2, where D is the duty cycle of the waveform and is defined as D=T1/(T1+T2).
Based on the above formulas, it is evident that when operating an electrosurgical generator in either the “cut”, “blend” or “coagulate” mode, the range of the crest factor varies from one mode to another. For example, the “cutting” mode typically entails generating an uninterrupted sinusoidal waveform in the frequency range of 100 kHz to 4 MHz with a crest factor in the range of 1.4 to 2.0. The “blend” mode typically entails generating an uninterrupted cut waveform with a duty cycle in the range of 25% to 75% and a crest factor in the range of 2.0 to 5.0. The “coagulate” mode typically entails generating an uninterrupted waveform with a duty cycle of approximately 10% or less and a crest factor in the range of 5.0 to 12.0. For the purposes herein, “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. “Vessel sealing” is defined as the process of liquefying the collagen in the tissue so that it reforms into a fused mass with significantly-reduced demarcation between the opposing tissue structures (opposing walls of the lumen). Coagulation of small vessels is usually sufficient to permanently close them. Larger vessels need to be sealed to assure permanent closure.
An increase in the crest factor results in more current per arc at a given power setting. Further, since tissue heating is proportional to the amount of current through the tissue squared and tissue vaporization is proportional to the amount of current being generated through the tissue, a doubling of current per arc results in four times as much tissue heating and twice the amount of tissue vaporization. Known electrosurgical generators do not control the crest factor of the electrosurgical output current. Such electrosurgical generators produce the same crest factor waveform across a range of tissue impedance. Some electrosurgical generators reduce or otherwise change the output power to achieve a surgical effect. However, no known electrosurgical generators alter both the output crest factor and output power across a range of tissue impedance to achieve a particular surgical effect. Accordingly, such electrosurgical generators do not have the ability to manipulate or control the proportion of tissue vaporization to tissue heating, in order to achieve more controllable and desirable surgical effects.
Therefore, it is an aspect of the present disclosure to provide an electrosurgical generator capable of regulating the output crest factor of the electrosurgical generator, as well as the output power, across a range of tissue impedance for controlling both tissue heating and tissue vaporization.